Vector Infection Determinants of Venezuelan Equine Encephalitis Virus Reside within the E2 Envelope Glycoprotein

Abstract

Epizootic subtype IAB and IC Venezuelan equine encephalitis viruses (VEEV) readily infect the epizootic mosquito vector Aedes taeniorhynchus. The inability of enzootic subtype IE viruses to infect this mosquito species provides a model system for the identification of natural viral determinants of vector infectivity. To map mosquito infection determinants, reciprocal chimeric viruses generated from epizootic subtype IAB and enzootic IE VEEV were tested for mosquito infectivity. Chimeras containing the IAB epizootic structural gene region and, more specifically, the IAB PE2 envelope glycoprotein E2 precursor gene demonstrated an efficient infection phenotype. Introduction of the PE2 gene from an enzootic subtype ID virus into an epizootic IAB or IC genetic backbone resulted in lower infection rates than those of the epizootic parent. The finding that the E2 envelope glycoprotein, the site of epitopes that define the enzootic and epizootic subtypes, also encodes mosquito infection determinants suggests that selection for efficient infection of epizootic mosquito vectors may mediate VEE emergence.Venezuelan equine encephalitis viruses (VEEV; Togaviridae family, Alphavirus genus) are single-stranded, enveloped, message-sense RNA viruses with genomes containing approximately 11,400 nucleotides (5, 9). Structural genes found in a 26S subgenomic message encode the capsid protein and the E2 and E1 envelope glycoproteins. During virus maturation, a PE2 precursor protein is cleaved into the E3 and E2 proteins; the E2 protein forms spikes on the surface of the virion (16), while the E1 protein lies parallel to the envelope (12). E2 is the site of the major antigenic determinants and probably interacts with cellular receptors. The nonstructural proteins responsible for viral replication are encoded by the nsP1 through nsP4 genes found in the 5Ј two-thirds of the genomic RNA (9) (Fig. 1).VEEV occur in two epidemiological settings: (i) a continuous, sylvatic enzootic cycle maintained between Culex (Melanoconion) spp. mosquito vectors and rodent reservoir hosts and (ii) an epidemic and/or epizootic cycle that involves several different mammalophilic mosquito species and exploits equines as highly efficient amplification hosts, resulting in extensive equine and human disease (29,32). Aedes (Ochlerotatus) taeniorhynchus (Reinert [18a] proposed that the subgenus Ochlerotaus be elevated to generic status; we do not believe that this change is justified until a more rigorous analysis is conducted) has been implicated as an epizootic vector during several VEE outbreaks (20,23,32,34).Of the six VEE antigenic subtypes (I through VI), only IAB and IC are etiologic agents of epidemics and/or epizootics because they generate equine viremia sufficient for efficient amplification (29, 32). Phylogenetic analyses indicate that epizootic VEEV have emerged repeatedly through mutations in enzootic subtype ID VEEV progenitors that enhance equine replication (18,32). The recent isolation from encephalitic horses during Mexican epizootics of sub...

Venezuelan equine encephalitis emergence: Enhanced vector infection from a single amino acid substitution in the envelope glycoprotein

“…Although the IE strains are now known not to be the progenitors of the IAB epizootic strains (1), Kramer and Scherer (21) demonstrated that this epizootic vector is more susceptible to IAB than to IE VEEV. This finding was later extended to the enzootic subtype ID progenitor and epizootic IC strains, and the E2 envelope glycoprotein precursor gene was shown to be responsible for the efficient infection phenotype of the IAB and IC strains (23). Our results extend these previous findings and address the specific amino acid residues involved in infectivity of this species.…”

“…1) but was significantly more efficient than enzootic strain 68U201 (P Ͻ 0.0001) after ingestion of high titered blood meals. This result identified the E2 envelope glycoprotein as a major site of subtype IE VEEV vector infectivity determinants and confirmed findings demonstrating the E2 gene region's role for increased vector infectivity of epizootic strains (23). Independent introduction of the E2-117-Lys or E2-407-Arg (IE.AAE2-E117K or IE.AAE2-S407R) epizootic mutations into the enzootic backbone generated viruses without significantly increased mosquito infec- tion phenotypes (P Ͼ 0.1) as compared with the 68U201 parental virus (Table 5 and Fig.…”

“…More recently, studies with reverse genetic methodology in which the E2 glycoprotein precursor genes (PE2) were exchanged between an enzootic IE strain and epizootic strains of subtype IAB or IC VEEV indicated the E2 glycoprotein is the major determinant for infection of O. taeniorhynchus (23). However, the presence of Ͼ70 amino acid differences in the PE2 (E2 precursor) genes of these subtypes precluded the identification specific residues in the vector infection phenotype.…”

“…Retrospective surveillance indicates that the proven epizootic mosquito vector, Ochlerotatus (formerly Aedes) taeniorhynchus, is the most abundant species in the affected communities during the rainy season, when the VEE outbreaks occurred. This species has been implicated as a vector during most major VEE epizootics (1) and is a more competent laboratory vector for epizootic IAB (21,22) and IC (23,24) viruses than for enzootic ID or IE viruses (23).…”

Venezuelan encephalitis emergence mediated by a phylogenetically predicted viral mutation

“…This finding suggests that ecological factors including transport of nascent epidemic strains to locations conducive to equine amplification, as well as epidemiologic and ecologic factors including equine herd immunity and mosquito densities, constrain the frequency of VEE epidemics rather than the generation of amplification-competent mutants. Envelope glycoprotein mutations that facilitate transmission by Aedes (Ochlerotatus) taeniorhynchus, an important mosquito vector, could also be involved in some epidemics (45,46). However, there is no evidence of enhanced vector infection by the 1992-1993 IC strains compared with the putative ID progenitors, such as strain ZPC738 (47).…”